1. Field Of The Invention
This invention relates generally to the art of removing permanent magnetization from objects.
2. Description Of The Prior Art
In the art of making reusable magnetic recordings, usually on the magnetizable surface of a magnetic recording tape, it is often desirable to "erase" the originally recorded data so that the magnetic recording tape or other medium can be used properly to make a second data recording in pure and accurate form, independent of the previously recorded data on the medium. In this respect, the term "erasing" must be distinguished from the term "degaussing". Degaussing is a more specific term relating to the returning of a magnetized object, such as a magnetic recording tape or other medium to the totally demagnetized state or an approximation thereof. A system which accomplishes this purpose has become known as a degausser. The term "erasing" is often employed generally to describe the obliterating of previously magnetically recorded data from a recording tape by any technique.
Two commonly employed techniques exist by which magnetically recorded data may be obliterated from a magnetic medium. The first technique, generally automatically used in purely digital, magnetically-saturated "computer" tapes, is simply to apply a strong, saturating, unvarying, uni-directional magnetic field along the recording length and direction of the tape by application of an "erase" magnetic head or permanent magnet. In this technique, the entire recorded length of a magnetic tape may simply be pulled across the active portion of such an "erase" head to accomplish obliteration of the originally recorded varying data. This procedure leaves all portions of the magnetic recording surface fully magnetized to saturation in the same direction, obliterating the previously recorded signal variations that comprise the recorded data. Different data may then be recorded on the tape by creating a magnetic field in the magnetic material forming the tape which is less than at saturation. This technique may be compared to scribbling over written material with a pencil to obliterate the written material and then writing something different with the eraser of the pencil.
The second technique for obliterating the data on a magnetic tape actually accomplishes degaussing. The degaussing technique attempts to completely remove both all previously recorded signals and any extraneous or structurally generated magnetic noise pulses that might remain in the magnetic coating due to residual magnetization of any sort. This degaussing technique and a subsequent writing operation onto the tape can be compared to erasing written material with a pencil eraser and then writing again over this spot. Degaussing includes the act of removing signal data plus any residual magnetic noise. On the other hand, the first technique described above does not necessarily or commonly remove any residual magnetic noise.
Degaussing is accomplished by applying to each portion of the recording medium surface a magnetic field which reverses direction a number of times and gradually decreases in absolute strength over the course of a degaussing cycle. The degaussing magnetic field is reduced gradually at each point of the tape surface from an initial level at least (usually more than) the level of apparent magnetic saturation through many cycles to a level gradually approaching zero. This leaves the magnetic recording surface only very slightly magnetized by the residual effects of the earth's magnetic field and any other residual fields applied by slight occasional magnetization of nearby equipment components.
Degaussing, sometimes called AC erasure, is much more effective than uni-directional saturation, sometimes called DC erasure, particularly with analog recording tapes, since degaussing reduces false, residual background noise to a practical minimum so that a subsequent recording of data will include as little noise as possible.
While simple DC saturation can be performed by running the tape across a fixed magnetic recording head supplied with a saturation strength direct current, degaussing can be accomplished by applying a high frequency peak saturation alternating current to the "erasing" head so that each portion of the magnetic tape experiences many alternating cycles of applied field which decrease gradually in intensity at a given point on the recording surface due to its mechanical withdrawal from the point of maximum magnetic action at the degaussing head active gap.
While degaussing of magnetic tapes by transportation across a degaussing head is highly effective, it is time consuming and not economical in that it requires the use of relatively expensive magnetic tape transportation devices to accomplish the desired purpose.
It is also known that full reels of magnetic tape can be degaussed, without unwinding and rewinding, in a "bulk" degaussing machine which applies the required, gradually-decreased alternating applied magnetic field to the entire tape roll without unwinding the tape.
Regardless of the particular degaussing machine, the end result of degaussing is to make the remanent flux remaining permanently on the recording surfaces as small as possible in order to leave the magnetic recording material in as nearly a totally demagnetized condition as possible. This result can be accomplished only by either raising the temperature of the magnetized recording material to a destructively high level or by causing the material to traverse a very large number of gradually shrinking magnetic hysteresis loops by periodically reversing the polarity of an applied magnetic field which gradually decreases in intensity.
In quality magnetic recording of analog signals, the generally accepted level of recorded signal and residual magnetic noise removal that must be accomplished by magnetic tape degaussing for both operational efficiency and reasonable security against unfriendly deliberate signal recovery is at least 90 db below the originally recorded saturation signal reproducible within a 10 Hz frequency bandwidth.
For special applications, it is sometimes desirable to reduce the residual magnetization of a previously recorded tape by more than 90 db below previous signal saturation.
In order to degauss a tape that had previously been recorded with a saturated signal, it is necessary to apply a magnetizing force equal to at least two and preferably three times the coercive force or coercivity of the magnetic recording material. Most magnetic tapes in high quality use have a coercivity ranging from 150 to 350 oersteds.
In more recent times, new varieties of magnetic recording tape and other magnetic media have been made available to the industry in which the coercivity has been increased substantially above the 350 oersted level. In the new, "high energy" magnetic recording media, the coercivity may approach 800 oersteds. It is not inconceivable that additional improvement in magnetic recording media will provide tapes with even higher coercivity values.
The appearance of especially high energy recording media requires the provision of degaussing systems capable of applying much higher magnetizing forces than have been practically possible with currently existing commercial degaussing systems, particularly bulk degaussing systems. While this problem is not especially difficult to overcome in tape degaussing systems which pass the entire length of the tape sequentially across a degaussing head, developing the required higher degaussing energy levels in a practical sense becomes more and more difficult when it is desired to use the more economical method of bulk degaussing described above.
The currently available best quality commercial bulk degaussing systems for magnetic tapes, such as, for example, the K-90 tape degausser manufactured by General Kinetics Incorporated, Rockville, Md., universally employ alternating current electromagnets to apply the required degaussing field from outside of the reel of tape or other package of magnetic medium to be degaussed. In order to achieve even the modest external field strengths required for ordinary, comparatively low energy magnetic tape, these degaussers use coils of heavy wire surrounding laminated transformer iron cores. These coil-core combinations generally are powered by continuous application of alternating current from a normal 60 or 50 Hz supply line. The useful portion of the magnetic field produced by iron cored coils of this type occurs in an air gap interrupting the iron core structure to allow insertion of the reel of magnetic tape. The reel of tape usually is inserted fully into the active air gap of such systems and either is caused to rotate while being slowly withdrawn geometrically from the field influence or is rotated at a fixed position in the air gap while the applied alternating electric field is gradually reduced. Either of these methods results in application through the sides of the tape roll of a gradually reducing applied alternating magnetic field required for degaussing.
As a result of the electromagnetic inefficiency of the necessary air gap in the coilcore structures thus employed, it has been necessary to apply very heavy continuous alternating electric currents to the coils, with resultant heating of the wire due to its electrical resistance and heating of the core material due to hysteresis energy loss in the iron laminations. In addition, it is usually necessary to employ large electrical capacitors connected in parallel with the bulk degausser coils to minimize the total amount of power line current required by providing a degree of power factor correction.
In a great many practical instances, the magnetic tapes to be bulk degaussed will be contained on reels provided with protective metal flanges. Since the externally applied magnetic field emanating from the iron core external coils must penetrate the metal flanges, eddy currents are induced in the flanges which cause them to react mechanically, producing noise, and occasionally vibrating sufficiently to damage the edges of the wound roll of tape. Due to eddy current losses, the metal reel flanges also become heated, with possible damaging results to the tape thereon.
This problem has been overcome by employing a coil of wire containing no iron core. The coil is essentially rectangular in cross section and is used with a reel of tape inserted at least partially within the confines of the inside of the coil with the axis of the coil lying parallel to the real flanges. Such a degaussing system is illustrated in U.S. Pat. No. 3,143,689 to Hall. In this patent, a storage capacitor is gradually charged from the rectified line current. The storage capacitor is periodically discharged through the degaussing coils in synchronism with the frequency of the line current. A second capacitor is connected in parallel with the degaussing coil to create a resonant circuit which rings after the energy in the storage capacitor has been applied to it. This creates a reversing magnetic field which gradually decreases in intensity in accordance with the time constant of the degaussing coil and the second capacitor. According to this patent, the storage capacitor should be discharged six or seven times to produce a sufficient erasure of the magnetic recording tape.
U.S. Pat. Nos. 3,321,586, 2,962,560, and 2,838,720 also teach degaussing systems which employ capacitor discharge circuitry.
As described above, the Hall patent relies upon the characteristics of a resonant circuit to control the frequency of the reversal of the magnetic field and the time period of the decay of the magnetic field. However, the characteristics of the resonant circuit are greatly affected by the nature of the magnetic material inserted within the coil. Thus, reels of different size, reels of the same size with different amounts of tape thereon, or reels of different material influence not only the frequency of field reversals, but also the time constant of the field decay.
Furthermore, in general, in view of the losses associated with the repeated generation of a magnetic field within the tape and surrounding air, it is impractical to extend the time constant of the resonant circuit to an extent sufficient to perform a complete degaussing. It is for this reason that Hall teaches that four or five repetitions are necessary to completely erase the tape.
The Hall patent also teaches the use of two orthogonally arranged sets of degaussing coils. Such an arrangement is necessary to uniformly erase all portions of the tape. This complicates not only the circuitry necessary to energize the coils, but also makes more difficult the problem of placing the tape within the orthogonal sets of coils.